Deproteinized natural rubber latex, method of preparing the same, rubber product using the same, and proteolytic agent for deproteinized natural rubber latex

ABSTRACT

The present invention provides a deproteinized natural rubber latex wherein coagulation of a rubber component does not occur when the concentration of calcium ions (Ca 2+ ) is 0.01 mol/L or less and coagulation of the rubber component occurs when the concentration of Ca 2+  is 0.1 mol/L or more; a method of preparing the deproteinized natural rubber latex, which comprises adding a protease and two or more surfactants having different coagulation properties to calcium ions (Ca 2+ ) to a natural rubber latex and maturing the natural rubber latex; a rubber product using the deproteinized natural rubber latex; and a proteolytic agent comprising a protease and two or more surfactants. The deproteinized natural rubber latex is a latex wherein rubber molecules are dispersed and stabilized by a surfactant, and also have good film forming properties by means of the anode coagulation method and is less likely to cause uneven thickness of the film and liquid dripping even when a mold is dipped repeatedly in a latex on formation of a rubber film.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a deproteinized natural rubberlatex, which is superior in balance between the film forming propertiesby means of the anode coagulation method and the dispersion stability ofa latex, a method of preparing the same, a rubber product using thedeproteinized natural rubber latex, and a proteolytic agent for naturalrubber latex.

[0002] Natural rubbers have widely been used in various fields, forexample, rubber gloves because of features such as large extension, highelasticity and strong film strength.

[0003] In the production of a glove made of a natural rubber, aproduction method is employed according to the thickness of a rubberfilm. A rubber glove having a film thickness of about 1 mm, for example,glove for home use is generally produced by a so-called anodecoagulation method of dipping a mold (hand mold) for glove, the surfaceof which is previously coated with a coagulant (anode coagulant), in anatural rubber latex.

[0004] It has recently been required for a rubber product using anatural rubber latex to highly remove a protein contained in theproduct. Main reasons include (1) immediate (I type) allergy such asdyspnea or urticaria is caused by bringing a natural rubber product intocontact with the skin or mucosa and a protein contained in a naturalrubber latex is considered to be a causative agent; (2) the protein cancause variations in quality and vulcanization properties of the naturalrubber product because the kind and quantity of the protein varydepending on the locality and production season of the latex; and (3)the protein can cause deterioration of mechanical characteristics suchas creep characteristics and aging resistance and electricalcharacteristics such as insulating properties of the rubber product.

[0005] Japanese Published Unexamined Patent (Kokai Tokkyo Koho Hei) No.6-56902 discloses a method of removing a protein and a decompositionproduct thereof through a series of the steps of adding a proteolyticenzyme (protease) and a surfactant to a natural rubber latex, maturingthe natural rubber latex, thereby decomposing a protein in the latex,and subjecting the latex to a centrifugation treatment. When subjectedto a deproteinization treatment according to this method, the protein inthe natural rubber latex can be removed in a very high level and thenitrogen content (N %) as measured by the Kjeldahl method is reduced to0.1% by weight or less.

[0006] For the purpose of preventing coagulation of a rubber componentcaused by an operation such as stirring by stabilizing a latexunstabilized as a result of removal of a protein, a surfactant isincorporated into a so-called deproteinized natural rubber latexobtained by the method disclosed in the publication described above.

[0007] The surfactant not only improves the mechanical stability of thelatex, but also exerts a large influence on the sensitivity to an anodecoagulant. When using a higher alcohol sulfate ester salt anionicsurfactant as the surfactant, the sensitivity of the latex to the anodecoagulant increases. Therefore, a film can be formed by the anodecoagulation method even under the same conditions as those in case ofthe non-deproteinized natural rubber latex.

[0008] However, because of too large sensitivity to the anode coagulant,the film is rapidly dried after dipping in the latex as compared withthe case of using the natural rubber latex. As a result, when a mold isrepeatedly dipped in the latex for the purpose of increasing thethickness of rubber film, there arise new problems such as uneventhickness of the rubber and liquid dripping.

[0009] When using a higher alkyl phenyl ether sulfate ester salt anionicsurfactant as the surfactant, the sensitivity of the latex to the anodecoagulant decreases and the rubber component is less likely to becoagulated. To form a film having nearly the same thickness as that incase of the non-deproteinized natural rubber by the anode coagulationmethod, there arise new problems that very special film formingconditions or a very complicated step are required.

[0010] The present applicants have previously found such a fact that adip product having a sufficient film thickness can be obtained by usinga specific heat sensitizer and a specific anode coagulant in a specificcombination and incorporating the specific combination in a large amountas compared with a conventional formulation (Japanese PublishedUnexamined Patent (Kokai Tokkyo Koho) No. 2000-17002).

[0011] However, there were problems, according to the method describedin the publication described above, since the both of the heatsensitizer and the anode coagulant are incorporated into the latex, thelatex becomes unstable as compared with a conventional heat sensitizingmethod using a natural rubber latex, thereby making it impossible toobtain long-term stability and making it hard to control heat-sensitiveproperties.

SUMMARY OF THE INVENTION

[0012] Thus, an object of the present invention is to provide adeproteinized natural rubber latex, which is a latex whose rubberparticles are dispersed and stabilized by a surfactant by means of adeproteinization treatment, and also which has good film formingproperties by means of the anode coagulation method and is capable ofproducing a rubber product having a sufficient film thickness under thesame film forming conditions as those of the prior art without causinguneven thickness of the film and liquid dripping even when a mold isdipped repeatedly in a latex on formation of a rubber film, and a methodof preparing the same.

[0013] Another object of the present invention is to provide a dipproduct using the deproteinized natural rubber latex by means of theanode coagulation method.

[0014] A still another object of the present invention is to provide aproteolytic agent, which is capable of subjecting a natural rubber latexto a high-level proteolyzation or deproteinization treatment and canalso impart sufficient coagulation properties by means of the anodecoagulation method and good film forming properties without causinguneven thickness and liquid dripping to the proteolyzed natural rubberlatex or deproteinized natural rubber latex obtained by the treatment,and also which can improve balance with the dispersion stability of thelatex.

[0015] As described above, a conventional deproteinized natural rubberlatex had such problems that too high sensitivity to the anode coagulantcaused uneven thickness of the rubber film and liquid dripping in caseof dipping repeatedly, or that too low sensitivity to the anodecoagulant made it difficult to coagulate the rubber component.

[0016] However, the present inventors have studied intensively to solvethe problems described above and found the following quite new fact.That is, even if the coagulation properties to calcium ions (hereinafterreferred to as “Ca²⁺”) of the deproteinized natural rubber latex arecontrolled and the latex is added dropwise in an aqueous solution havingthe concentration of Ca²⁺ of 0.01 mol/L or less, coagulation of therubber component in the latex does not occur. However, when thecoagulation properties are controlled so that coagulation of the rubbercomponent occurs when the latex is added dropwise in an aqueous solutionhaving the concentration of Ca²⁺ of 0.1 mol/L or more, the sensitivityto anode coagulants such as calcium nitrate, calcium chloride and thelike becomes proper surprisingly, there by making it possible tomarkedly improve the film forming properties by means of the anodecoagulation method and to afford good balance with the dispersionstability of the latex. Thus, the present invention has been completed.

[0017] The present invention is directed to:

[0018] (I) a deproteinized natural rubber latex which is obtained bysubjecting to a treatment for decomposition and removal of a protein,wherein coagulation of a rubber component does not occur when theconcentration of calcium ions (Ca²⁺) is 0.01 mol/L or less andcoagulation of the rubber component occurs when the concentration ofCa²⁺ is 0.1 mol/L or more;

[0019] (II) a method of preparing a deproteinized natural rubber latex,which comprises adding a protease and two or more surfactant shavingdifferent coagulation properties to calcium ions (Ca²⁺) to a naturalrubber latex, and maturing the natural rubber latex, wherein two or moresurfactants are stably dispersed when the concentration of Ca²⁺ of anaqueous solution (25° C.) containing the surfactants is 0.1 mol/L orless, and are coagulated when Ca²⁺ of the aqueous solution is 1.0 mol/Lor more;

[0020] (III) a rubber product using a deproteinized natural rubberlatex, which is obtained by dipping a dipping mold, the surface of whichis coated with an anode coagulant, in the deproteinized natural rubberlatex containing a vulcanizing agent added therein described in the term(I), vulcanizing a rubber film formed on the surface of the dippingmold, and removing the rubber film from the dipping mold; and

[0021] (IV) a proteolytic agent for natural rubber latex, comprising aprotease and two or more surfactants having different coagulationproperties to calcium ions (Ca²⁺), wherein two or more surfactants arestably dispersed when the concentration of Ca²⁺ of an aqueous solution(25° C.) containing the surfactants is 0.1 mol/L or less, and arecoagulated when Ca²⁺ of the aqueous solution is 1.0 mol/L or more.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention includes the respective inventions relatingto the following deproteinized natural rubber latex, the method ofpreparing the same, the rubber product using the same, and theproteolytic agent for deproteinized natural rubber latex.

[0023] (1) A deproteinized natural rubber latex which is obtained bysubjecting to a treatment for decomposition and removal of a protein,wherein coagulation of a rubber component does not occur when theconcentration of calcium ions (Ca²⁺) is 0.01 mol/L or less andcoagulation of the rubber component occurs when the concentration ofcalcium ions (Ca²⁺) is 0.1 mol/L or more.

[0024] (2) The deproteinized natural rubber latex described in the term(1), wherein the treatment for decomposition of the protein is conductedby adding a protease and two or more surfactants having differentcoagulation properties to calcium ions (Ca²⁺) to a natural rubber latexand maturing the natural rubber latex, and two or more surfactants arestably dispersed when the concentration of Ca²⁺ of an aqueous solution(25° C.) containing the surfactants is 0.1 mol/L or less, and arecoagulated when Ca²⁺ of the aqueous solution is 1.0 mol/L or more.

[0025] (3) The deproteinized natural rubber latex described in the term(1), wherein the deproteinized natural rubber latex, which is obtainedby subjecting to a treatment for decomposition and removal of a protein,is prepared by adding a protease to a natural rubber latex and maturingthe natural rubber latex, centrifuging the latex, thereby to isolate acreamy rubber solid content, and dispersing the rubber solid content inan aqueous medium, wherein the aqueous medium contains two or moresurfactants having different coagulation properties to calcium ions(Ca²⁺) and enables the surfactants to stably disperse when a liquidtemperature is 25° C. and the concentration of calcium ions (Ca²⁺) is0.1 mol/L or less, and to coagulate when the liquid temperature is 25°C. and the concentration of Ca²⁺ is 1.0 mol/L or more.

[0026] (4) The deproteinized natural rubber latex described in the term(2) or (3), wherein two or more surfactants include:

[0027] at least one surfactant (sometimes referred to as “surfactant H”)selected from the group consisting of carboxylic acid anionicsurfactant, higher alcohol sulfate ester salt anionic surfactant,sulfonic acid anionic surfactant and phosphoric acid anionic surfactant,and

[0028] at least one surfactant (sometimes referred to as “surfactant L”)selected from the group consisting of higher alkylphenyl ether sulfateester salt anionic surfactant and higher alkyl ether sulfate ester saltanionic surfactant.

[0029] (5) The deproteinized natural rubber latex described in the term(2), wherein the total amount of two or more surfactants added ontreatment for decomposition of a protein is within a range from 0.01 to10 parts by weight based on 100 parts by weight of the rubber solidcontent of the natural rubber latex.

[0030] (6) The deproteinized natural rubber latex described in the term(3), wherein the total content of two or more surfactants in the aqueousdispersion medium is within a range from 0.01 to 10 parts by weightbased on 100 parts by weight of the solid content dispersed in theaqueous dispersion medium.

[0031] (7) A method of preparing a deproteinized natural rubber latex,which comprises adding a protease and two or more surfactants havingdifferent coagulation properties to calcium ions (Ca²⁺) to a naturalrubber latex, and maturing the natural rubber latex, wherein two or moresurfactants are stably dispersed when the concentration of Ca²⁺ of anaqueous solution (25° C.) containing the surfactants is 0.1 mol/L orless, and are coagulated when Ca²⁺ of the aqueous solution is 1.0 mol/Lor more.

[0032] (8) A method of preparing a natural rubber latex, which comprisessubjecting a natural rubber latex to a treatment for decomposition of aprotein due to a protease and a treatment for removal of a protein dueto centrifugation, and dispersing the resulting creamy rubber solidcontent in an aqueous medium, wherein the aqueous medium contains two ormore surfactants having different coagulation properties to calcium ions(Ca²⁺) and enables the surfactants to stably disperse when a liquidtemperature is 25° C. and the concentration of Ca²⁺ is 0.1 mol/L orless, and to coagulate when the liquid temperature is 25° C. and theconcentration of Ca²⁺ is 1.0 mol/L or more.

[0033] (9) A rubber product using a deproteinized natural rubber latex,which is obtained by dipping a dipping mold, the surface of which iscoated with an anode coagulant, in the deproteinized natural rubberlatex containing a vulcanizing agent added therein described in any oneof terms (1) to (6), vulcanizing a rubber film formed on the surface ofthe dipping mold, and removing the rubber film from the dipping mold.

[0034] (10) A proteolytic agent for natural rubber latex, comprising aprotease and two or more surfactant shaving different coagulationproperties to calcium ions (Ca²⁺), wherein two or more surfactants arestably dispersed when the concentration of Ca²⁺ of an aqueous solution(25° C.) containing the surfactants is 0.1 mol/L or less, and arecoagulated when Ca²⁺ of the aqueous solution is 1.0 mol/L or more.

[0035] (11) The proteolytic agent for natural rubber latex described inthe term (10), wherein two or more surfactants include:

[0036] at least one surfactant H selected from the group consisting ofcarboxylic acid anionic surfactant, higher alcohol sulfate ester saltanionic surfactant, sulfonic acid anionic surfactant and phosphoric acidanionic surfactant, and

[0037] at least one surfactant L selected from the group consisting ofhigher alkyl phenyl ether sulfate ester salt anionic surfactant andhigher alkyl ether sulfate ester salt anionic surfactant.

[0038] (12) The proteolytic agent for natural rubber latex described inthe term (11), wherein a ratio of the content of the surfactant H to thesurfactant L is within a range from 15:85 to 70:30 by weight ratio.

[0039] According to the invention (1), it is made possible to produce arubber product having a sufficient film thickness under the sameconditions as those in case of forming a film from a non-deproteinizednatural rubber latex by means of the anode coagulation method withoutcausing uneven thickness of the rubber film and liquid dripping when amold is repeatedly dipped.

[0040] In case of the deproteinized natural rubber latex described inJapanese Published Unexamined Patent (Kokai Tokkyo Koho) No. 2000-17002,there arises a problem that long-term stability of the latex is impairedby incorporating the both of the heat sensitizer and the anode coagulantin the amount larger than that in case of a conventional formulation asdescribed above. However, according to the deproteinized natural rubberlatex of the present invention, such a problem is not likely to arise.

[0041] As used herein, the expression “coagulation of the rubber occurs”refers to the fact that the rubber component in the deproteinizednatural rubber latex is completed isolated in the upper layer of thelatex in the form of a creamy solid content (in such a case, an aqueoussolution containing Ca²⁺ becomes transparent) or the fact that thecreamy solid content is partially observed in the deproteinized naturalrubber latex (in such a case, the aqueous solution containing Ca²⁺ isstill in the state of white turbidity).

[0042] As used herein, the expression “coagulation of the rubber doesnot occur” refers to the fact that the rubber component in thedeproteinized natural rubber latex is maintained in the dispersed andsuspended state and the creamy rubber solid content is not observed inthe latex (in such a case, the aqueous solution containing Ca²⁺ is stillin the state of white turbidity).

[0043] The presence or absence of “coagulation of the rubber component”is judged whether or not the rubber component in a deproteinized naturalrubber latex is observed in the form of a creamy solid content afteradding dropwise the latex in an aqueous solution wherein theconcentration of Ca²⁺ is controlled to a predetermined value. In case“coagulation of the rubber component does not occur”, the rubbercomponent is rapidly dispersed in the aqueous solution containing Ca²⁺when the latex is added dropwise.

[0044] Regarding the natural rubber latex used to evaluate thecoagulation properties to Ca²⁺, the concentration of the rubber solidcontent is not specifically limited. However, since it becomes difficultto judge coagulation of the rubber component when the concentration ofthe rubber solid content is too low, the concentration of the solidrubber content of the latex is preferably set within a range from about30 to 60% by weight beforehand. The liquid temperature of the aqueoussolution having a predetermined concentration of Ca²⁺ used to evaluatethe coagulation properties to Ca²⁺ of the latex is not specificallylimited, but is preferably set to a fixed temperature of about 25° C. Inthe present invention, those prepared by controlling the concentrationof the rubber solid content to 60% by weight were used as the latex incase of evaluating the coagulation properties to Ca²⁺ and the liquidtemperature of the aqueous solution having a predetermined concentrationof Ca²⁺ was set to 25° C., unless otherwise specified.

[0045] The deproteinized natural rubber latex of the invention (1) canbe prepared by stabilizing the latex using a combination of two or moresurfactants having different coagulation properties to Ca²⁺. Thecoagulation properties to the Ca²⁺ of the deproteinized natural rubberlatex is set within a predetermined range by the combination of thesesurfactants.

[0046] More specifically, the deproteinized natural rubber latex of thepresent invention (1) can be prepared by the process (i) of subjecting anatural rubber latex to a deproteinization treatment using two or moresurfactants having different coagulation properties to Ca²⁺ and aprotease, or the process (ii) of dispersing a natural rubber subjectedpreviously to a deproteinization treatment by various conventionallyknown methods in an aqueous dispersion medium wherein the coagulationproperties to Ca²⁺ are set within a predetermined range.

[0047] In the deproteinized natural rubber latex of the presentinvention, two or more surfactants having different coagulationproperties (dispersion stability) to Ca²⁺ are used for the purpose ofmaintaining the dispersed state of the rubber component unstabilized asa result of the deproteinization treatment.

[0048] In case the coagulation properties to the concentration of Ca²⁺of the latex are set within the above range using a combination of asurfactant having relatively high coagulation properties (lowdispersibility) to Ca²⁺ and a surfactant having relatively lowcoagulation properties (high dispersibility) to Ca²⁺, thestably-dispersed state of the rubber component can be maintained withoutcausing coagulation of the rubber component in the latex at the stagebefore film formation by means of the anode coagulation method. Specificexamples of the stage before film formation by means of the anodecoagulation method include storage of the deproteinized natural rubberlatex for a long period of time, application of mechanical vibration tothe latex during conveying, and addition of various additives such asvulcanizing agent to the latex.

[0049] When the deproteinized natural rubber latex of the presentinvention is brought into contact with an anode coagulant having theconcentration used conventionally in the anode coagulation method,coagulation occurs in the surfactant having higher coagulationproperties to Ca²⁺ between two surfactants and the dispersibility of therubber in the latex is drastically lowered. Therefore, it is madepossible to form a rubber film by means of the anode coagulation method.

[0050] As used herein, the expression “two or more surfactants arecoagulated” refers to the fact that at least one of two surfactants isbonded with Ca²⁺ to form a salt insoluble in water. As used herein, theexpression “two or more surfactants are stably dispersed” refers to thefact that the dispersibility of the surfactants in the aqueous solutionis maintained without forming a salt as a result of bonding with Ca²⁺.

[0051] As used herein, the term “aqueous dispersion medium” mainlyrefers to water. As far as the dispersion stability of the latex are notadversely affected, the aqueous dispersion medium include those whichcontain other solvents miscible with water (for example, organicsolvent) and additives incorporated conventionally in the latex used infilm formation by means of the anode coagulation method.

[0052] The coagulation properties to Ca²⁺ of the surfactant can be setwithin the above range by using those belonging to the surfactant H incombination with those belonging to the surfactant L as two or moresurfactants in the inventions (2) and (3).

[0053] Balance between the film forming properties by means of the anodecoagulation method and the dispersion stability with respect to thedeproteinized natural rubber latex can be further improved by settingthe total amount or total content of two or more surfactants to theabove range in the inventions (5) and (6).

[0054] According to the inventions (7) and (8), it is made possible toprepare a deproteinized natural rubber latex wherein coagulation of arubber component does not occur when the concentration of calcium ions(Ca²⁺) is 0.01 mol/L or less and coagulation of the rubber componentoccurs when the concentration of Ca²⁺ is 0.1 mol/L or more. The methodof preparing the deproteinized natural rubber latex of the invention (7)is one aspect of the method of preparing the deproteinized naturalrubber latex according to the invention (2).

[0055] The method of preparing the deproteinized natural rubber latex ofthe invention (8) is one aspect of the method of preparing thedeproteinized natural rubber latex according to the invention (3).

[0056] In the methods of preparing the deproteinized natural rubberlatex according to the inventions (7) and (8), as two or moresurfactants, those including at least one surfactant H selected from thegroup consisting of carboxylic acid anionic surfactant, higher alcoholsulfate ester salt anionic surfactant, sulfonic acid anionic surfactantand phosphoric acid anionic surfactant, and at least one surfactant Lselected from the group consisting of higher alkyl phenyl ether sulfateester salt anionic surfactant and higher alkyl ether sulfate ester saltanionic surfactant are preferably used.

[0057] In the invention (7), the total amount of two or more surfactantsadded on treatment for decomposition of a protein is preferably within arange from 0.01 to 10 parts by weight based on 100 parts by weight ofthe rubber solid content of the natural rubber latex. In the invention(8), the total content of two or more surfactants in the aqueousdispersion medium is preferably within a range from 0.01 to 10 parts byweight based on 100 parts by weight of the rubber solid contentdispersed in the aqueous dispersion medium.

[0058] In such a case, balance between the film forming properties bymeans of the anode coagulation method and the dispersion stability of alatex can be further improved.

[0059] The rubber product of the invention (9) is produced by adding avulcanizing agent to the deproteinized natural rubber latex of thepresent invention, dipping a dipping mold, the surface of which iscoated with an anode coagulant, in the deproteinized natural rubberlatex, vulcanizing a rubber film formed on the surface of the dippingmold, and removing the rubber film from the dipping mold.

[0060] According to the rubber product using the deproteinized naturalrubber latex of the invention (9) and the method of producing the same,it is made possible to produce a rubber product made of a deproteinizednatural rubber latex, wherein a fear of the occurrence of immediate (Itype) allergy has been markedly reduced by the deproteinizationtreatment, by using a non-deproteinized natural rubber latex under thesame conditions. Accordingly, the invention (9) is suited forapplication to a rubber glove for home use having a thickness of about 1mm.

[0061] As used herein, the term “rubber product” refers to a rubberproduct produced by the anode coagulation method and specific examplesthereof include medical appliances (for example, catheter, doubleballoon, etc.), finger cots and toys, including rubber gloves.

[0062] In the rubber product using the deproteinized natural rubberlatex of the present invention and the method of producing the same,examples of the anode coagulant include, but are not limited to, metalsalts having an ionic value of 2 or more and organic alkylamine salts.Examples of the metal salt having an ionic value of 2 or more includecalcium nitrate and calcium chloride. These anode coagulants aregenerally used in the form of an aqueous solution.

[0063] Regarding the proteolytic agents for deproteinized natural rubberlatex of the inventions (11) to (12), the degree of the coagulation toCa²⁺ of the surfactant contained in the proteolytic agent is controlledwithin the above range, as described above. Therefore, in case thetreatment for decomposition of a protein in the natural rubber latex isconducted using the proteolytic agent of the present invention, properanode coagulation properties (in other words, excellent film formingproperties by means of the anode coagulation method) can be imparted tothe latex after subjecting to the treatment.

[0064] Regarding the proteolyzed natural rubber latex (or deproteinizednatural rubber latex) obtained by such a treatment, the stably-dispersedstate of the rubber component can be maintained without causingcoagulation of the rubber component in the latex at the stage beforefilm formation by means of the anode coagulation method. Specificexamples of the stage before film formation by means of the anodecoagulation method include storage of the deproteinized natural rubberlatex for a long period of time, application of mechanical vibration tothe latex during conveying, and addition of various additives such asvulcanizing agents to the latex.

[0065] When the proteolyzed natural rubber latex (or deproteinizednatural rubber latex) of the present invention is brought into contactwith an anode coagulant having the concentration used conventionally inthe anode coagulation method, coagulation occurs in the surfactanthaving higher coagulation properties to Ca²⁺among two surfactants andthe dispersibility of the rubber in the latex is drastically lowered.Therefore, it is made possible to form a rubber film by means of theanode coagulation method.

[0066] Since the proteolytic agent of the present invention can controlthe sensitivity to anode coagulants such as calcium nitrate and calciumchloride of the proteolyzed natural rubber latex (or deproteinizednatural rubber latex) treated with the treating agent to propersensitivity, the proteolytic agent of the present invention is suited toprepare a proteolyzed natural rubber latex (or deproteinized naturalrubber latex) for film formation by means of the anode coagulationmethod.

[0067] In the proteolytic agent for natural rubber latex of the presentinvention, two or more surfactants are preferably those including atleast one surfactant H selected from the group consisting of carboxylicacid anionic surfactant, higher alcohol sulfate ester salt anionicsurfactant, sulfonic acid anionic surfactant and phosphoric acid anionicsurfactant, and at least one surfactant L selected from the groupconsisting of higher alkyl phenyl ether sulfate ester salt anionicsurfactant and higher alkyl ether sulfate ester salt anionic surfactantare preferably used.

[0068] The proteolytic agent for natural rubber latex of the presentinvention contains two or more surfactants having different coagulationproperties (dispersion stability) to Ca²⁺, the degree of coagulation toCa²⁺ being controlled within a predetermined range. As described above,when using those having relatively high coagulation properties (lowdispersibility) to Ca²⁺ in combination with those having relatively lowcoagulation properties (high dispersibility) to Ca²⁺, the coagulationproperties to Ca²⁺ of the latex can be easily set within the rangedescribed above. Specific examples of the surfactant having relativelyhigh coagulation properties to Ca²⁺ include those included in the groupof the surfactant H, while specific examples of the surfactant havingrelatively low coagulation properties to Ca²⁺ include those included inthe group of the surfactant L.

[0069] The coagulation properties to Ca²⁺ with respect to two or moresurfactants are usually evaluated by adding dropwise the surfactant inthe form of an aqueous solution to an aqueous solution containing Ca²⁺.In this case, the concentration of the aqueous surfactant is notspecifically limited. However, since it becomes difficult to judge thepresence or absence of coagulation of the rubber component when theconcentration is too low, it is preferred to previously set theconcentration of the aqueous solution of two or more surfactants(mixture) to about 10% by weight. The liquid temperature of the aqueoussolution having a predetermined concentration of Ca²⁺ used to evaluatethe coagulation properties to Ca²⁺ of the latex is not specificallylimited, but is preferably set within a temperature range where the filmforming treatment is conducted by the anode coagulation method. Ingeneral, liquid temperature of the aqueous solution is preferably set toabout 25° C. in case of evaluating the degree of coagulation propertiesto the concentration of Ca²⁺. In the present invention, the liquidtemperature of the aqueous solution having a predetermined concentrationof Ca²⁺ was set to 25° C., unless otherwise specified.

[0070] In the proteolytic agent for natural rubber latex of the presentinvention, a ratio of the content of the surfactant H to the surfactantL is preferably within a range from 15:85 to 70:30 in a weight ratio.

[0071] The use of the surfactant H and the surfactant L in a weightratio within a range from 15:85 to 70:30 makes it possible to easilycontrol the degree of the coagulation properties to the Ca²⁺ of thesurfactant. Accordingly, it is made possible to improve balance betweenthe film forming properties by means of the anode coagulation method andthe storage stability of the latex itself with respect to theproteolyzed natural rubber latex (or deproteinized natural rubber latex)treated with the proteolytic agent of the present invention.

[0072] Embodiments of the present invention will now be described.

[0073] [Deproteinized natural rubber latex and method of preparing thesame]

[0074] (Natural rubber latex)

[0075] The natural rubber latex used to prepare the deproteinizednatural rubber latex of the present invention may be any of a fieldlatex obtained as a rubber sap and an ammonia-retained concentratedlatex.

[0076] (Protease)

[0077] In the present invention, the protease used in a treatment fordecomposition of a protein to the natural rubber latex is notspecifically limited and various conventionally known proteases can beused and, for example, an alkaline protease is preferable. The proteasemay be derived from any of bacteria, filamentous bacteria and yeast, andthe protease is preferably derived from bacteria, particularlypreferably from the genus Bacillus. It is also possible to use enzymessuch as lipase, esterase, amylase, lacase and cellulase in combination.

[0078] When using the alkaline protease, its activity [measured valueobtained by modification of the Anson-hemoglobin method (Anson. M. L. J.Gen. Physiol., 22, 79 (1938))] is within a range from 0.1 to 50 APU/g,and preferably within a range from 1 to 25 APU/g.

[0079] The amount of the protease varies depending on the activity ofthe protease itself, and is not specifically limited. In general, thecontent of the protease is preferably controlled within a range from0.0001 to 20 parts by weight, and more preferably within a range from0.001 to 10 parts by weight, based on 100 parts by weight of the rubbercomponent in the natural rubber latex. When the content of the proteaseis within the range described above, a protein in the latex can besufficiently decomposed while maintaining the activity of the protease.Alternatively, the effect corresponding to the amount of the proteasecan be exerted effectively and, therefore, it is advantageous in view ofthe cost.

[0080] (Surfactant)

[0081] The surfactant used to prepare the deproteinized natural rubberlatex of the present invention is composed of a combination of two ormore surfactants having different coagulation properties to Ca²⁺. It isrequired for the combination of two or more surfactants to be set sothat the coagulation properties to Ca²⁺ with respect to the aqueoussolution containing two or more surfactants are within a predeterminedrange.

[0082] Specifically, it is required that, when a liquid temperature ofan aqueous solution (or aqueous dispersion medium) containing two ormore surfactants is 25° C. and the concentration of Ca²⁺ is 0.1 mol/L orless, the surfactants are stably dispersed, whereas, when the liquidtemperature of the aqueous solution (or aqueous dispersion medium) is25° C. and the concentration of Ca²⁺ is 1.0 mol/L or more, thesurfactants are coagulated.

[0083] As the surfactant used in the present invention, for example,

[0084] at least one surfactant H selected from the group consisting ofcarboxylic acid anionic surfactant, higher alcohol sulfate ester saltanionic surfactant, sulfonic acid anionic surfactant and phosphoric acidanionic surfactant, and

[0085] at least one surfactant L selected from the group consisting ofhigher alkyl phenyl ether sulfate ester salt anionic surfactant andhigher alkyl ether sulfate ester salt anionic surfactant may be used incombination.

[0086] Those included in the group of the surfactant H are surfactantshaving relatively high coagulation properties (relatively lowdispersibility) to Ca²⁺, and those included in the group of thesurfactant L are surfactants having relatively low coagulationproperties (relatively high dispersibility) to Ca²⁺.

[0087] Specific examples of those included in the group of thesurfactant H are shown in Table 1. Also, specific examples of thoseincluded in the group of the surfactant L are shown in Table 2. TABLE 1*Surfactant H (having high coagulation properties to Ca²⁺) No. Kind andname of surfactants H-1 Carboxylic acid anionic surfactant H-1-1Potassium oleate H-1-2 Sodium dialkylsuccinate H-1-3 Sodium oleate H-1-4Sodium laurate H-1-5 Sodium stearate H-2 Higher alcohol sulfate estersalt anionic surfactant H-2-1 Sodium laurate H-2-2 Sodium cetyl sulfateH-2-3 sodium stearyl sulfate H-2-4 Sodium oleyl sulfate H-3 Sulfonicacid anionic surfactant H-3-1 Sodium dodecylbenzene sulfonate H-4Phosphoric acid anionic surfactant H-4-1 Potassium polyoxyethylenenonylphenyl phosphate

[0088] TABLE 2 *Surfactant L (having low coagulation properties to Ca²⁺)No. Kind and name of surfactants L-1 Higher alkyl phenyl ether sulfateester anionic surfactant L-1-1 Sodium POE nonyl phenyl ether sulfate L-2Higher alkyl ether sulfate ester salt anionic surfactant L-2-1 SodiumPOE alkyl ether sulfate

[0089] Although a mixing ratio of the surfactant H to the surfactant Lis not specifically limited, a weight ratio (H:L) is preferably setwithin a range from 15:85 to 70:30.

[0090] When the total amount (total content) of the surfactant H and thesurfactant L is equal to 100, the lower limit of the addition amount(content) of the surfactant H is preferably 20 [H:L (weightratio)=20:80], and more preferably 25 [H:L (weight ratio)=25:75], withinthe range described above. On the other hand, the upper limit of thecontent of the surfactant H is preferably 65 [H:L (weight ratio) 65:35],and more preferably 60 [H:L (weight ratio) =60:40], within the rangedescribed above.

[0091] (Content of protease and surfactant)

[0092] In the present invention, especially second invention among thepresent invention, the content of the protease and that of thesurfactant on treatment for removal of a protein are not specificallylimited. To efficiently promote the treatment for decomposition of aprotein, a ratio of the both is preferably set within a range from 1:1to 1:200, and more preferably from 1:10 to 1:50.

[0093] (Method of treatment for decomposition and removal of protein)

[0094] The treatment for decomposition of a protein to a natural rubberlatex is conducted by adding a predetermined amount of the protease anda predetermined combination of the surfactants to a natural rubber as araw material and maturing the mixture for about several tens of minutesto one week, and preferably about 1 to 3 days.

[0095] This maturing treatment may be conducted while stirring the latexor allowing it to stand. If necessary, the temperature may becontrolled. The temperature is preferably controlled within a range from5 to 90° C., and more preferably from 20 to 60° C. to obtain sufficientactivity of the enzyme. When the temperature is lower than 5° C., thereis a fear that the enzyme reaction does not proceed. On the other hand,when the temperature exceeds 90° C., there is a fear that the enzyme isdevitalized.

[0096] The treatment for removal of a protein (or decomposition productthereof) after the decomposition of a protein includes, but is notspecifically limited to, a treatment of concentrating the latex bycentrifugation or ultrafiltration and separating the non-rubbercomponent transferred in water such as protein decomposition product andthe rubber particles in the latex, or a treatment of separating therubber particles by coagulation using an acid. Among these treatments,purification by centrifugation is most preferred in view of the accuracyand efficiency of purification.

[0097] The protease added to the natural rubber latex is washed andremoved by the above purification treatment after being subjected to thetreatment for removal of a protein. Regarding the surfactants added tothe natural rubber latex, a portion of them is washed and removed by thepurification treatment. Although a portion of the surfactants isremained in the deproteinized natural rubber latex even after thepurification treatment and acts as a stabilizer of the latex, thestability of the deproteinized natural rubber latex is drasticallyimpaired when the residual amount is too small (when almost all of themis removed by the purification treatment). However, when the treatmentof washing (purification) of the latex after the deproteinizationtreatment is conducted by a conventional centrifugation method underconventional treatment conditions, that is, a washing (purification)treatment is conducted under the conditions where the protease and thedecomposition product of the protein can be washed and removed, it isnot necessary to add a new surfactant to the latex after thedeproteinization treatment.

[0098] More specifically, in case the cleaning (purification) treatmentis conducted by a centrifugation treatment, sufficient stability andheat-sensitive coagulation properties of the latex can be maintained bydispersing a cream component separated in the upper layer bycentrifugation under the conditions of 5,000 to 14,000 rpm for about 1to 60 minutes again in water having almost the same volume as that ofthe cream component even after the deproteinization treatment using thewater-soluble polymer which is previously added before thedeproteinization treatment.

[0099] In case of preparing a deproteinized natural rubber latex of thethird invention among the present invention, it is necessary tosufficiently take notice of the amount of the surfactant used in thetreatment for decomposition of a protein to the natural rubber latex.The reason is as follows. That is, when the surfactant remains in acoagulated rubber obtained by the deproteinization treatment, it islikely to exert an adverse influence on the anode coagulation properties(sensitivity to anode coagulant) of the deproteinized natural rubberlatex. In case of the deproteinized natural rubber latex of the thirdinvention, since the latex is prepared by dispersing the solid rubbercomponent obtained by the deproteinization treatment in the aqueoussolution containing a predetermined surfactant, any problem does notarise even if the rubber component is coagulated on deproteinizationtreatment. Accordingly, if the protein and decomposition product thereofcan be sufficiently removed, the surfactant may not be added ondeproteinization treatment.

[0100] (Degree of deproteinization)

[0101] Although the degree of the deproteinization in the deproteinizednatural rubber latex used in the present invention is not specificallylimited, it is required to control so that the nitrogen content (N %) asdetermined by the Kjeldahl method after the deproteinization treatmentis 0.1% or less, preferably 0.05% or less, and more preferably 0.02% orless, in order to make a final rubber product low-allergic. When thenitrogen content exceeds the above range, there is a fear that theoccurrence of the allergic reaction cannot be sufficiently suppressed inuse of a final rubber product because of insufficient degree of thedeproteinization.

[0102] The degree of the deproteinization can also be confirmed by thepresence or absence of adsorption and degree of adsorption on the basisof the protein by means of an infrared absorption spectrum. In therubber treated by using the proteolytic agent of the present invention,an absorption at 3320 cm⁻¹ derived from short-chain peptide or aminoacid may be observed. However, it is preferable that an absorption at3280 cm⁻¹ derived from polymer peptide as a cause for allergy is small.It is more preferable that any absorption at 3280 cm⁻¹ is not observed.

[0103] [Rubber product using deproteinized natural rubber latex]

[0104] The rubber product using the deproteinized natural rubber latexof the present invention is obtained by dipping a dipping mold, thesurface of which is coated with an anode coagulant previously, in thedeproteinized natural rubber latex of the present invention to form arubber film formed on the surface of the dipping mold, vulcanizing therubber film, and removing the rubber film from the dipping mold.

[0105] Examples of the anode coagulant, with which the surface of thedipping mold is previously coated, include, but are not limited to,metal salts having an ionic value of 2 or more and organic alkylaminesalts. Examples of the metal salt having an ionic value of 2 or moreinclude calcium nitrate and calcium chloride. These anode coagulants aregenerally used in the form of an aqueous solution.

[0106] The concentration of the anode coagulant on film formation maybeset according to a conventional method and is not specifically limited,but is usually set within a range from 5 to 20% by weight, andpreferably from 10 to 15% by weight. This concentration is within arange from about 0.3 to 1.2 mol/L, and preferably from about 0.6 to 0.9mol/L, when reduced based on the concentration of Ca²⁺ assuming that theanode coagulant is calcium nitrate (molecular weight: 164).

[0107] The dipping mold used in the production of the rubber productvaries depending on the shape of the objective rubber product. Forexample, when the rubber product is a rubber glove, a conventionallyknown hand mold may be used as the dipping mold.

[0108] The film forming conditions may be set by a conventional methodaccording to the kind of the objective rubber product and the thicknessof the rubber film.

[0109] Vulcanizing agents are previously added to the deproteinizednatural rubber latex used in the production of the rubber productsdescribed above. If necessary, various additives, for example,vulcanization chemicals such as vulcanization accelerators, auxiliaryvulcanization accelerators and vulcanization retardants, and variousadditives such as fillers can be incorporated.

[0110] Examples of the vulcanizing agent include sulfur, and organicsulfur-containing compounds such as trimethylthiourea andN,N′-diethylthiourea, and these vulcanizing agents can be used alone orin combination. The amount of the vulcanizing agent is decided by evenbalance between the vulcanization degree and the amount of thevulcanization accelerator, but is usually set within a range from 0.1 to5 parts by weight, and preferably from 0.5 to 2 parts by weight, basedon 100 parts by weight of the rubber solid content in the rubber latex.

[0111] Examples of the vulcanization accelerator include zincN-ethyl-N-phenyldithiocarbamate (PX), zinc dimethyldithiocarbamate (PZ),zinc diethyldithiocarbamate (EZ), zinc dibutyldithiocarbamate (BZ), zincsalt of 2-mercaptobenzothiazole (MZ) and tetramethylthiuram disulfide(TT). These vulcanization accelerators can be used alone or incombination. The amount of the vulcanization accelerator is preferablyset within a range from about 0.5 to 3 parts by weight based on 100parts by weight of the rubber solid content in the rubber latex.Examples of the auxiliary vulcanization accelerator include zinc white.The amount of the auxiliary vulcanization accelerator is preferably setwithin a range from about 0.5 to 3 parts by weight based on 100 parts byweight of the rubber solid content in the rubber latex.

[0112] Examples of the filler include kaolin clay, hard clay and calciumcarbonate. The amount of the filler is preferably 10 parts by weight orless based on 100 parts by weight of the rubber solid content in therubber latex.

[0113] It is not necessary to incorporate a specific heat sensitizer anda specific anode coagulant into the deproteinized natural rubber latexin a larger amount as compared with a conventional formulation, like thedeproteinized natural rubber latex described Japanese PublishedUnexamined Patent (Kokai Tokkyo Koho) No. 2000-17002. Therefore,according to the rubber product using the deproteinized natural rubberlatex of the present invention, it is made possible to prepare ahigh-quality rubber product, which is less contaminated with impurities,while reducing the material cost.

EXAMPLES

[0114] The following Examples and Comparative Examples furtherillustrate the present invention.

Example 1

[0115] (1) Preparation of deproteinized natural rubber latex

[0116] A high ammonia latex of a natural rubber was diluted so that theconcentration of the rubber component becomes 30% by weight. Adeproteinizing agent comprising a protease and a surfactant was added inthe amount of 1% by weight based on the rubber content of the latex.Then, the mixture was matured by standing while maintaining the liquidtemperature at 30° C. for 24 hours, thereby subjecting to a treatmentfor decomposition of a protein.

[0117] As the deproteinizing agent, a mixture of 2 parts by weight of aprotease [alkali protease, manufactured by Novo-Nordisk Bioindustri A/Sunder the trade name of “Alcalase 2.0M”], 49 parts by weight ofpotassium oleate [surfactant H (No. H-1-1) shown in Table 1] and 49parts by weight of sodium polyoxyethylene nonyl phenyl ether sulfate[surfactant L (No. L-1-1) shown in Table 2] was used.

[0118] After the completion of the treatment for decomposition of aprotein, the latex was subjected to a centrifugation treatment at 13,000rpm for 30 minutes and the cream component separated in the upper layerwas dispersed again in water having the same volume as that of the creamcomponent to obtain a deproteinized natural rubber latex.

[0119] (2) Formation of rubber film (production of rubber glove)

[0120] Based on 100 parts by weight of the rubber solid content in thedeproteinized natural rubber latex, 1 part by weight of colloidal sulfurdispersed in water, 0.5 parts by weight of zinc white and 1 part byweight of a vulcanization accelerator (zinc dibutyldithiocarbamate (BZ),manufactured by OUCHISHINKO CHEMICAL INDUSTRIAL CO., LTD. under thetrade name of “NOCCELER Bz”) were added, followed by maturing(pre-vulcanization) at 4° C. for 24 hours.

[0121] After previously coating the surface of a glass dipping mold(hand mold) with an aqueous 15 wt % solution of calcium nitrate (anodecoagulant), this mold was dipped in the vulcanized latex for 10 secondsto form a rubber film on the surface of the mold.

[0122] After forming the rubber film, the mold drawn up from thevulcanized latex was allowed to stand at room temperature (about 25° C.)for 60 seconds and dipped again (double dipping) in the vulcanized latexfor 10 seconds.

[0123] Furthermore, the rubber film formed on the surface of the moldwas vulcanized by heating to 100° C. and the rubber film was removedfrom the dipping mold to obtain a rubber product (rubber glove).

Example 2

[0124] In the same manner as in Example 1, except that a mixture of 2parts by weight of an alkali protease, 24 parts by weight of potassiumoleate [surfactant H (No. H-1-1) and 74 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the deproteinizing agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Example 3

[0125] In the same manner as in Example 1, except that sodium laurylsulfate [surfactant H (No. H-2-1) shown in Table 1] was used as thesurfactant H in place of potassium oleate, “(1) Preparation ofdeproteinized natural rubber latex” and “(2) Formation of rubber film”were conducted.

Example 4

[0126] In the same manner as in Example 1, except that a mixture of 2parts by weight of an alkali protease, 24 parts by weight of sodiumlauryl sulfate [surfactant H (No. H-2-1) and 74 parts by weight ofsodium polyoxyethylene alkyl ether sulfate [surfactant L (No. L-1-2)shown in Table 2] was used as the deproteinizing agent, “(1) Preparationof deproteinized natural rubber latex” and “(2) Formation of rubberfilm” were conducted.

Example 5

[0127] In the same manner as in Example 1, except that 2 parts by weightof an alkali protease, 49 parts by weight of sodium lauryl sulfate[surfactant H (No. H-2-1) and sodium polyoxyethylene alkyl ether sulfate[surfactant L (No. L-1-2)] were used as the deproteinizing agent, “(1)Preparation of deproteinized natural rubber latex” and “(2) Formation ofrubber film” were conducted.

Comparative Example 1

[0128] In the same manner as in Example 1, except that 2 parts by weightof an alkali protease and 98 parts by weight of potassium oleate[surfactant H (No. H-1-1) were used as the deproteinizing agent (thatis, the surfactant H was used alone), “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 2

[0129] In the same manner as in Example 1, except that 2 parts by weightof an alkali protease and 98 parts by weight of sodium polyoxyethylenenonyl phenyl ether sulfate [surfactant L (No. L-1-1) were used as thedeproteinizing agent (that is, the surfactant L was used alone), “(1)Preparation of deproteinized natural rubber latex” and “(2) Formation ofrubber film” were conducted.

Comparative Example 3

[0130] In the same manner as in Example 1, except that a mixture of 2parts by weight of an alkali protease, 74 parts by weight of potassiumoleate [surfactant H (No. H-1-1) and 24 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the deproteinizing agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 4

[0131] In the same manner as in Example 1, except that a mixture of 2parts by weight of an alkali protease, 12 parts by weight of potassiumoleate [surfactant H (No. H-1-1) and 86 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

[0132] (Evaluation of coagulation properties to Ca²⁺)

[0133] (i) With respect to the surfactant used in the treatment fordecomposition of a protein in Examples 1 to 5 and Comparative Examples 1to 4, the coagulation properties to Ca²⁺ were evaluated. Evaluation wasconducted in the following procedure. That is, the surfactant used inthe treatment for decomposition of a protein was dissolved in water inthe same content as that in the deproteinizing agent, thereby to controlthe concentration to 10% by weight, and then an aqueous solution of thesurfactant was added dropwise in an aqueous solution (25° C.) containingCa²⁺. The concentration of Ca²⁺ measured includes 0.1 mol/L and 1.0mol/L. The evaluation results are as shown in Table 3.

[0134] Each content of the surfactant shown in Table 3 is an approximatevalue which shows the content of the surfactant in the deproteinizingagent shown in Table 4.

[0135] (ii) With respect to the deproteinized natural rubber latexesobtained in Examples 1 to 5 and Comparative Examples 1 to 4, thecoagulation properties to Ca²⁺ were evaluated. Evaluation was conductedin the following procedure. That is, the concentration of the rubbersolid content in the deproteinized natural rubber latex was controlledto 60% by weight, and then the latex was added dropwise in an aqueoussolution (25° C.) containing Ca²⁺. The concentration of Ca²⁺ measuredincludes 0.01 mol/L and 0.1 mol/L. The evaluation results are as shownin Table 4.

[0136] (Measurement of nitrogen content)

[0137] With respect to the deproteinized natural rubber latexes obtainedin Examples 1 to 5 and Comparative Examples 1 to 4, the nitrogen content(N %) was measured by the Kjeldahl method. The measurement results areas shown in the column of “N %” in Table 4.

[0138] (Evaluation of physical properties of vulcanized rubber film)

[0139] With respect to the rubber products (rubber products) obtained inthe Examples 1 to 5 and Comparative Examples 1 to 4, the thickness anduniformity of the rubber film were evaluated. In accordance with thetest procedure defined in JIS K 6301, the tensile strength TB (MPa) andthe elongation at break EB (%) were determined. The measurement resultsare as shown in Table 5. TABLE 3 *Surfactant used in treatment fordecomposition of protein Coagulation Content properties to Ca²⁺ ofsurfactant 0.1 mol/L 1.0 mol/L Example 1 H-1-1   50% dispersedcoagulated L-1-1   50% Example 2 H-1-1 24.5% dispersed coagulated L-1-175.5% Example 3 H-2-1   50% dispersed coagulated L-1-1   50% Example 4H-2-1 24.5% dispersed coagulated L-1-2 75.5% Example 5 H-2-1   50%dispersed coagulated L-1-2   50% Comp. Example 1 H-1-1  100% coagulatedcoagulated Comp. Example 2 L-1-1  100% dispersed dispersed Comp. Example3 H-1-1 75.5% coagulated coagulated L-1-1 24.5% Comp. Example 4 H-1-112.2% dispersed dispersed L-1-1 87.8%

[0140] TABLE 4 Content of each component of Characteristics ofdeproteinized natural deproteinizing rubber latex agent (weightCoagulation properties to Ca²⁺ ratio) N% 0.01 mol/L 0.1 mol/L Example 1Protease 2 0.019 non-coagulated coagulated H-1-1 49 L-1-1 49 Example 2Protease 2 0.021 non-coagulated coagulated H-1-1 24 L-1-1 74 Example 3Protease 2 0.018 non-coagulated coagulated H-2-1 49 L-1-1 49 Example 4Protease 2 0.019 non-coagulated coagulated H-2-1 24 L-1-2 74 Example 5Protease 2 0.020 non-coagulated coagulated H-2-1 49 L-1-2 49 Comp.Protease 2 0.019 coagulated coagulated Example 1 H-1-1 98 Comp. Protease2 0.020 non-coagulated non-coagulated Example 2 L-1-1 98 Comp. Protease2 0.020 coagulated coagulated Example 3 H-1-1 74 L-1-1 24 Comp. Protease2 0.019 non-coagulated non-coagulated Example 4 H-1-1 12 L-1-1 86

[0141] TABLE 5 Properties of vulcanized rubber film Film TensileElongation thickness Film strength at break (mm) uniformity T_(B) (MPa)E_(B) (%) Example 1 0.38 good 27.4 960 Example 2 0.37 good 27.1 950Example 3 0.37 good 26.8 950 Example 4 0.37 good 27.7 970 Example 5 0.38good 26.8 950 Comp. 0.40 poor 27.8 980 Example 1 Comp. 0.32 poor 27.1960 Example 2 Comp. 0.39 poor 27.1 940 Example 3 Comp. 0.33 poor 27.3950 Example 4

[0142] As is apparent from Table 3 to Table 5, regarding alldeproteinized natural rubber latexes obtained in Examples 1 to 5, thatis, the deproteinized natural rubber latexes obtained by subjecting tothe treatment for decomposition of a protein using the surfactant Hhaving relatively high coagulation properties to Ca²⁺ in combinationwith the surfactant L having relatively low coagulation properties toCa²⁺, a rubber component was not coagulated when the concentration ofcalcium ions (Ca²⁺) is 0.01 mol/L or less and the rubber component wascoagulated when the concentration of Ca²⁺ is 0.1 mol/L or more.

[0143] Therefore, it has been found that the deproteinized naturalrubber latexes obtained in Examples 1 to 5 are suited to form a film bythe anode coagulation method and are capable of conducting filmformation of a rubber product having a sufficient film thickness by theanode coagulation method, as is apparent from the results of “filmthickness” and “film uniformity”.

[0144] To the contrary, in case Comparative Example 1 wherein only thesurfactant H was used and Comparative Example 3 wherein the content ofthe surfactant H was extremely large, since the rubber component wascoagulated when the concentration of Ca²⁺ is 0.01 mol/L or less,sufficient film formation could not conducted, thereby causing a problemsuch as poor uniformity of the film.

[0145] In case Comparative Example 2 wherein only the surfactant L wasused and Comparative Example 4 wherein the content of the surfactant Lwas extremely larger than that of the surfactant H, since the rubbercomponent was not coagulated even when the concentration of Ca²⁺ is 0.1mol/L or more, sufficient film formation could not conducted, therebycausing such a problem that a film having a sufficient film thicknesscan not be uniformly formed.

Example 6

[0146] (1) Preparation of deproteinized natural rubber latex

[0147] A high ammonia latex of a natural rubber was diluted so that theconcentration of the rubber component becomes 30% by weight. Aproteolytic agent comprising a protease and a surfactant was added inthe amount of 1% by weight based on the rubber content of the latex.Then, the mixture was matured by standing while maintaining the liquidtemperature at 30° C. for 24 hours, thereby subjecting to a treatmentfor decomposition of a protein.

[0148] As the proteolytic agent, a mixture of 2 parts by weight of aprotease [alkali protease, manufactured by Novo-Nordisk Bioindustri A/Sunder the trade name of “Alcalase 2.0M”], 49 parts by weight ofpotassium oleate [surfactant H (No. H-1-1) shown in Table 1] and 49parts by weight of sodium polyoxyethylene nonyl phenyl ether sulfate[surfactant L (No. L-1-1) shown in Table 2] was used.

[0149] After the completion of the treatment for decomposition of aprotein, the latex was subjected to a centrifugation treatment at 13,000rpm for 30 minutes and the cream component separated in the upper layerwas dispersed again in water having the same volume as that of the creamcomponent to obtain a deproteinized natural rubber latex.

[0150] (2) Formation of rubber film

[0151] Based on 100 parts by weight of the rubber solid content in thedeproteinized natural rubber latex, 1 part by weight of colloidal sulfurdispersed in water, 0.5 parts by weight of zinc white and 1 part byweight of a vulcanization accelerator (zinc dibutyldithiocarbamate (BZ),manufactured by OUCHISHINKO CHEMICAL INDUSTRIAL CO., LTD. under thetrade name of “NOCCELER Bz”) were added, followed by maturing(pre-vulcanization) at 40° C. for 24 hours.

[0152] After previously coating the surface of a glass dipping mold(hand mold) with an aqueous 15 wt % solution of calcium nitrate (anodecoagulant), this mold was dipped in the vulcanized latex for 10 secondsto form a rubber film on the surface of the mold.

[0153] After forming the rubber film, the mold drawn up from thevulcanized latex was allowed to stand at room temperature (about 25° C.)for 60 seconds and dipped again (double dipping) in the vulcanized latexfor 10 seconds.

[0154] Furthermore, the rubber film formed on the surface of the moldwas vulcanized by heating to 100° C. and the rubber film was removedfrom the dipping mold to obtain a rubber product.

Example 7

[0155] In the same manner as in Example 6, except that a mixture of 2parts by weight of an alkali protease, 24 parts by weight of potassiumoleate [surfactant H (No. H-1-1) and 74 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Example 8

[0156] In the same manner as in Example 6, except that sodium laurylsulfate [surfactant H (No. H-2-1) shown in Table 1] was used as thesurfactant H in place of potassium oleate, “(1) Preparation ofdeproteinized natural rubber latex” and “(2) Formation of rubber film”were conducted.

Example 9

[0157] (1) Preparation of deproteinized natural rubber latex

[0158] The same amount of the proteolytic agent as that used in Example6 was added to a high ammonia latex of a natural rubber diluted so thatthe concentration of the rubber component becomes 30% by weight and thetreatment for decomposition of a protein was conducted under the sameconditions.

[0159] After the completion of the treatment, the resulting proteolyzednatural rubber latex was used as it is without subjecting to thetreatment for removal of a decomposition product due to a centrifugationtreatment.

[0160] (2) Formation of rubber film and evaluation of film formingproperties and physical properties

[0161] In the same manner as in Example 6, except that the proteolyzednatural rubber latex was used in place of the deproteinized naturalrubber latex, “(2) Formation of rubber film” was conducted.

Example 10

[0162] In the same manner as in Example 9, except that a mixture (whichis the same proteolytic agent as that of Example 7) of 2 parts by weightof an alkali protease, 24 parts by weight of potassium oleate[surfactant H (No. H-1-1) and 74 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Example 11

[0163] In the same manner as in Example 9, except that a mixture (whichis the same proteolytic agent as that of Example 8) of 2 parts by weightof an alkali protease, 24 parts by weight of sodium lauryl sulfate[surfactant H (No. H-2-1) and 74 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 5

[0164] In the same manner as in Example 6, except that 2 parts by weightof an alkali protease and 98 parts by weight of potassium oleate[surfactant H (No. H-1-1) were used as the proteolytic agent (that is,the surfactant H was used alone), “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 6

[0165] In the same manner as in Example 6, except that 2 parts by weightof an alkali protease and 98 parts by weight of sodium polyoxyethylenenonyl phenyl ether sulfate [surfactant L (No. L-1-1) were used as theproteolytic agent (that is, the surfactant L was used alone), “(1)Preparation of deproteinized natural rubber latex” and “(2) Formation ofrubber film” were conducted.

Comparative Example 7

[0166] In the same manner as in Example 6, except that a mixture of 2parts by weight of an alkali protease, 74 parts by weight of potassiumoleate [surfactant H (No.H-1-1) and 24 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 8

[0167] In the same manner as in Example 6, except that a mixture of 2parts by weight of an alkali protease, 12 parts by weight of potassiumoleate [surfactant H (No. H-1-1) and 86 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant L (No. L-1-1)]was used as the proteolytic agent, “(1) Preparation of deproteinizednatural rubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 9

[0168] In the same manner as in Example 9, except that a mixture (whichis the same proteolytic agent as that of Comparative Example 5) of 2parts by weight of an alkali protease and 98 parts by weight ofpotassium oleate [surfactant H (No. H-1-1) was used as the proteolyticagent, “(1) Preparation of deproteinized natural rubber latex” and “(2)Formation of rubber film” were conducted.

Comparative Example 10

[0169] In the same manner as in Example 9, except that a mixture (whichis the same proteolytic agent as that of Comparative Example 6) of 2parts by weight of an alkali protease and 98 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant H (No. L-1-1) wasused as the proteolytic agent, “(1) Preparation of deproteinized naturalrubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 11

[0170] In the same manner as in Example 9, except that a mixture (whichis the same proteolytic agent as that of Comparative Example 7) of 2parts by weight of an alkali protease, 74 parts by weight of potassiumoleate [surfactant H (No. H-1-1) and 24 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant H (No. L-1-1) wasused as the proteolytic agent, “(1) Preparation of deproteinized naturalrubber latex” and “(2) Formation of rubber film” were conducted.

Comparative Example 12

[0171] In the same manner as in Example 9, except that a mixture (whichis the same proteolytic agent as that of Comparative Example 8) of 2parts by weight of an alkali protease, 12 parts by weight of potassiumoleate [surfactant H (No. H-1-1) and 86 parts by weight of sodiumpolyoxyethylene nonyl phenyl ether sulfate [surfactant H (No. L-1-1) wasused as the proteolytic agent, “(1) Preparation of deproteinized naturalrubber latex” and “(2) Formation of rubber film” were conducted.

[0172] (Evaluation of coagulation properties to Ca²⁺)

[0173] (i) With respect to the surfactant used in the treatment fordecomposition of a protein in Examples 6 to 11 and Comparative Examples5 to 12, the coagulation properties to Ca²⁺ with respect to thesurfactant contained in the proteolytic agent were evaluated. Evaluationwas conducted in the following procedure. That is, the surfactant usedin the treatment for decomposition of a protein was dissolved in waterin the same content as that in the proteolytic agent, thereby to controlthe concentration to 10% by weight, and then an aqueous solution of thesurfactant was added dropwise in an aqueous solution (25° C.) containingCa²⁺. The concentration of Ca²⁺ measured includes 0.1 mol/L and 1.0mol/L. The evaluation results are as shown in Table 6.

[0174] Each content of the surfactant shown in Table 6 is an approximatevalue which shows the content of the surfactant in the proteolytic agentshown in Table 7 and Table 8.

[0175] (ii) With respect to the deproteinized natural rubber latexesobtained in Examples 6 to 11 and Comparative Examples 5 to 12, thecoagulation properties to Ca²⁺ were evaluated. Evaluation was conductedin the following procedure. That is, the concentration of the rubbersolid content in the deproteinized natural rubber latex was controlledto 60% by weight, and then the latex was added dropwise in an aqueoussolution (25° C.) containing Ca²⁺. The concentration of Ca²⁺ measuredincludes 0.01 mol/L and 0.1 mol/L. The evaluation results are as shownin Table 7 and Table 8.

[0176] From a viewpoint of achievement of good balance between the filmforming properties by means of the anode coagulation method and thedispersion stability of the latex itself, the coagulation properties toCa²⁺ with respect to the deproteinized natural rubber latex orproteolyzed natural rubber latex are preferably those whereincoagulation of a rubber component does not occur when the concentrationof Ca²⁺ is 0.01 mol/L or less and coagulation of the rubber componentoccurs when the concentration of Ca²⁺ is 0.1 mol/L or more.

[0177] (Measurement of nitrogen content)

[0178] With respect to the deproteinized natural rubber latexes obtainedin Examples 6 to 8 and Comparative Examples 5 to 8, the nitrogen content(N %) was measured by the Kjeldahl method. The measurement results areas shown in the column of “N %” in Table 7.

[0179] (Evaluation of physical properties of vulcanized rubber film)

[0180] With respect to the rubber products obtained in the Examples 6 to11 and Comparative Examples 5 to 12, the thickness and uniformity of therubber film were evaluated. In accordance with the test proceduredefined in JIS K 6301, the tensile strength TB (MPa) and the elongationat break EB (%) were determined. The measurement results are as shown inTable 9. TABLE 6 *Surfactant used in treatment for decomposition ofprotein Coagulation Content of properties to Ca2+ surfactant 0.1 mol/L1.0 mol/L Example 6 & 9 H-1-1   50% dispersed coagulated L-1-1   50%Example 7 & 10 H-1-1 24.5% dispersed coagulated L-1-1 75.5% Example 8 &11 H-2-1   50% dispersed coagulated L-1-1   50% Comp. Example 5 & 9H-1-1  100% coagulated coagulated Comp. Example 2 & 10 L-1-1  100%dispersed dispersed Comp. Example 7 & 11 H-1-1 75.5% coagulatedcoagulated L-1-1 24.5% Comp. Example 8 & 12 H-1-1 12.2% disperseddispersed L-1-1 87.8%

[0181] TABLE 7 Content of each component of Characteristics ofdeproteinized natural proteolytic rubber latex agent Coagulationproperties to Ca²⁺ (weight ratio) N% 0.01 mol/L 0.1 mol/L Example 6Protease 2 0.019 non-coagulated coagulated H-1-1 49 L-1-1 49 Example 7Protease 2 0.021 non-coagulated coagulated H-1-1 24 L-1-1 74 Example 8Protease 2 0.018 non-coagulated coagulated H-2-1 49 L-1-1 49 Comp.Protease 2 0.019 coagulated coagulated Example 5 H-1-1 98 Comp. Protease2 0.020 non-coagulated non-coagulated Example 6 L-1-1 98 Comp. Protease2 0.020 coagulated coagulated Example 7 H-1-1 74 L-1-1 24 Comp. Protease2 0.019 non-coagulated non-coagulated Example 8 H-1-1 12 L-1-1 86

[0182] TABLE 8 Content of each component of Characteristics ofdeproteinized proteolytic natural rubber latex agent Coagulationproperties to Ca²⁺ (weight ratio) N% 0.01 mol/L 0.1 mol/L Example 9Protease 2 — non-coagulated coagulated H-1-1 49 L-1-1 49 ExampleProtease 2 — non-coagulated coagulated 10 H-1-1 24 L-1-1 74 ExampleProtease 2 — non-coagulated coagulated 11 H-2-1 49 L-1-1 49 Comp.Protease 2 — coagulated coagulated Example 9 H-1-1 98 Comp. Protease 2 —non-coagulated non-coagulated Example L-1-1 98 10 Comp. Protease 2 —coagulated coagulated Example H-1-1 74 11 L-1-1 24 Comp. Protease 2 —non-coagulated non-coagulated Example H-1-1 12 12 L-1-1 86

[0183] TABLE 9 Film Tensile Elongation thickness Film strength at break(mm) uniformity T_(B) (MPa) E_(B) (%) Example 6 0.38 good 27.4 960Example 7 0.37 good 27.1 950 Example 8 0.37 good 26.8 950 Comp. 0.40poor 27.8 980 Example 5 Comp. 0.32 poor 27.1 960 Example 6 Comp. 0.39poor 27.1 940 Example 7 Comp. 0.33 poor 27.3 950 Example 8 Example 90.38 good 26.5 920 Example 10 0.37 good 25.7 930 Example 11 0.37 good25.8 930 Comp. 0.38 poor 26.0 910 Example 9 Comp. 0.36 poor 24.1 960Example 10 Comp. 0.39 poor 25.1 940 Example 11 Comp. 0.31 poor 24.3 920Example 12

[0184] As is apparent from Table 6 to Table 9, regarding all proteolyzednatural rubber latexes or deproteinized natural rubber latexes obtainedby using the proteolytic agents of Examples 6 to 11 (using thesurfactant H having relatively high coagulation properties to Ca²⁺ incombination with the surfactant L having relatively low coagulationproperties) to Ca²⁺, coagulation did not occur when the concentration ofCa²⁺ is 0.01 mol/L or less and coagulation occurred when theconcentration of Ca²⁺ is 0.1 mol/L or more.

[0185] Therefore, it has been found that the proteolytic agents ofExamples 6 to 11 are suited to form a film by the anode coagulationmethod and are capable of conducting film formation of a rubber producthaving a sufficient film thickness by the anode coagulation method, asis apparent from the results of “film thickness” and “film uniformity”.

[0186] To the contrary, in case Comparative Examples 1 and 5 whereinonly the surfactant H was used and Comparative Examples 7 and 11 whereinthe content of the surfactant H was extremely larger than that of thesurfactant L, since the coagulation occurred when the concentration ofCa²⁺ is 0.01 mol/L, sufficient film formation could not conducted,thereby causing a problem such as poor uniformity of the film.

[0187] In case Comparative Examples 6 and 10 wherein only the surfactantL was used and Comparative Examples 8 and 12 wherein the content of thesurfactant L was extremely larger than that of the surfactant H, sincecoagulation did not occurred even when the concentration of Ca²⁺ is 0.1mol/L, sufficient film formation could not conducted, thereby causingsuch a problem that a film having a sufficient film thickness can not beuniformly formed.

What is claimed is:
 1. A deproteinized natural rubber latex which isobtained by subjecting to a treatment for decomposition and removal of aprotein, wherein coagulation of a rubber component does not occur whenthe concentration of calcium ions (Ca²⁺) is 0.01 mol/L or less andcoagulation of the rubber component occurs when the concentration ofCa²⁺ is 0.1 mol/L or more.
 2. The deproteinized natural rubber latexaccording to claim 1, wherein the treatment for decomposition of aprotein is conducted by adding a protease and two or more surfactantshaving different coagulation properties to calcium ions (Ca²⁺) to anatural rubber latex and maturing the natural rubber latex, and the twoor more surfactants are stably dispersed when the concentration of Ca²⁺of an aqueous solution (25° C.) containing the surfactants is 0.1 mol/Lor less, and are coagulated when Ca²⁺ of the aqueous solution is 1.0mol/L or more.
 3. The deproteinized natural rubber latex according toclaim 1, wherein the deproteinized natural rubber latex, which isobtained by subjecting to a treatment for decomposition and removal of aprotein, is prepared by adding a protease to a natural rubber latex andmaturing the natural rubber latex, centrifuging the latex, thereby toisolate a creamy rubber solid content, and dispersing the rubber solidcontent in an aqueous medium, wherein the aqueous medium contains two ormore surfactants having different coagulation properties to calcium ions(Ca²⁺), and enables the surfactants to stably disperse when a liquidtemperature is 25° C. and the concentration of Ca²⁺ is 0.1 mol/L orless, and to coagulate when the liquid temperature is 25° C. and theconcentration of Ca²⁺ is 1.0 mol/L or more.
 4. The deproteinized naturalrubber latex according to claim 2 or 3, wherein the two or moresurfactants include: at least one surfactant H selected from the groupconsisting of carboxylic acid anionic surfactant, higher alcohol sulfateester salt anionic surfactant, sulfonic acid anionic surfactant andphosphoric acid anionic surfactant, and at least one surfactant Lselected from the group consisting of higher alkyl phenyl ether sulfateester salt anionic surfactant and higher alkyl ether sulfate ester saltanionic surfactant.
 5. The deproteinized natural rubber latex accordingto claim 2, wherein the total amount of two or more surfactants added ontreatment for decomposition of a protein is within a range from 0.01 to10 parts by weight based on 100 parts by weight of the rubber solidcontent of the natural rubber latex.
 6. The deproteinized natural rubberlatex according to claim 3, wherein the total content of the two or moresurfactants in the aqueous dispersion medium is within a range from 0.01to 10 parts by weight based on 100 parts by weight of the rubber solidcontent dispersed in the aqueous dispersion medium.
 7. A method ofpreparing a deproteinized natural rubber latex, which comprises adding aprotease and two or more surfactants having different coagulationproperties to calcium ions (Ca²⁺) to a natural rubber latex, andmaturing the natural rubber latex, wherein the two or more surfactantsare stably dispersed when the concentration of Ca²⁺ of an aqueoussolution (25° C.) containing the surfactants is 0.1 mol/L or less, andare coagulated when Ca²⁺ of the aqueous solution is 1.0 mol/L or more.8. A method of preparing a natural rubber latex, which comprisessubjecting a natural rubber latex to a treatment for decomposition of aprotein due to a protease and a treatment for removal of a protein dueto centrifugation, and dispersing the resulting creamy rubber solidcontent in an aqueous medium, wherein the aqueous medium contains two ormore surfactants having different coagulation properties to Ca²⁺ to anatural rubber latex, and enables the surfactants to stably dispersewhen a liquid temperature is 25° C. and the concentration of Ca²⁺ is 0.1mol/L or less, and to coagulate when the liquid temperature is 25° C.and the concentration of Ca²⁺ is 1.0 mol/L or more.
 9. A rubber productusing a deproteinized natural rubber latex, which is obtained by dippinga dipping mold, the surface of which is coated with an anode coagulant,in the deproteinized natural rubber latex containing a vulcanizing agentadded therein of any one of claims 1 to 6, vulcanizing a rubber filmformed on the surface of the dipping mold, and removing the rubber filmfrom the dipping mold.
 10. A proteolytic agent for natural rubber latex,comprising a protease and two or more surfactants having differentcoagulation properties to calcium ions (Ca²⁺), wherein the two or moresurfactants are stably dispersed when the concentration of Ca²⁺ of anaqueous solution (25° C.) containing the surfactants is 0.1 mol/L orless, and are coagulated when Ca²⁺ of the aqueous solution is 1.0 mol/Lor more.
 11. The proteolytic agent for natural rubber latex according toclaim 10, wherein the two or more surfactants include: at least onesurfactant H selected from the group consisting of carboxylic acidanionic surfactant, higher alcohol sulfate ester salt anionicsurfactant, sulfonic acid anionic surfactant and phosphoric acid anionicsurfactant, and at least one surfactant L selected from the groupconsisting of higher alkyl phenyl ether sulfate ester salt anionicsurfactant and higher alkyl ether sulfate ester salt anionic surfactant.12. The proteolytic agent for natural rubber latex according to claim11, wherein a ratio of the content of the surfactant H to the surfactantL is within a range from 15:85 to 70:30 by weight ratio.